Instead of
using the default crypt
method, use the more secure SHA-512 method of password encryption,
which also allows passwords longer than 8 characters. It is also
necessary to change the obsolete /var/spool/mail location for user mailboxes that
Shadow uses by default to the /var/mail location used currently:

6.55.2. Configuring
Shadow

This package contains utilities to add, modify, and delete users
and groups; set and change their passwords; and perform other
administrative tasks. For a full explanation of what password shadowing means, see the
doc/HOWTO file within the unpacked
source tree. If using Shadow support, keep in mind that programs
which need to verify passwords (display managers, FTP programs,
pop3 daemons, etc.) must be Shadow-compliant. That is, they need to
be able to work with shadowed passwords.

To enable shadowed passwords, run the following command:

pwconv

To enable shadowed group passwords, run:

grpconv

Shadow's stock configuration for the useradd utility has a few caveats
that need some explanation. First, the default action for the
useradd utility is to
create the user and a group of the same name as the user. By
default the user ID (UID) and group ID (GID) numbers will begin
with 1000. This means if you don't pass parameters to useradd, each user will be a
member of a unique group on the system. If this behaviour is
undesireable, you'll need to pass the -g parameter to useradd. The default parameters
are stored in the /etc/default/useradd file. You may need to modify
two parameters in this file to suit your particular needs.

/etc/default/useradd Parameter
Explanations

GROUP=1000

This parameter sets the beginning of the group numbers used
in the /etc/group file. You can modify it to anything you
desire. Note that useradd will never reuse a
UID or GID. If the number identified in this parameter is
used, it will use the next available number after this. Note
also that if you don't have a group 1000 on your system the
first time you use useradd without the
-g parameter, you'll
get a message displayed on the terminal that says:
useradd: unknown GID
1000. You may disregard this message and group number
1000 will be used.

CREATE_MAIL_SPOOL=yes

This parameter causes useradd to create a mailbox
file for the newly created user. useradd will make the group
ownership of this file to the mail group with 0660 permissions. If you
would prefer that these mailbox files are not created by
useradd, issue
the following command: